Corrosion Protection for Offshore Pipelines

Methods for external protection of subsea pipelines

  • Design plays a big role in corrosion control
  • Primary protection is provided by coatings
  • Cathodic protection is used to protect holidays in the coating

Internal Corrosion Control Requires Monitoring

  • Chemical injection
  • Corrosion inhibitors
  • Biocides
  • Scale inhibitors
  • Materials selection
  • 13-Cr alloys
  • Duplex Stainless steels
  • Not usually controlled with a coating

Corrosion Control – Integrity Management Flow Diagram

Corrosion Control – Integrity Management Flow Diagram
Fig. 1: Corrosion Control – Integrity Management Flow Diagram

The operating temperature has a great influence on corrosion control.

The operating temperature can be as high as 65oC to 100oC

For every increase of 10oC, the corrosion rate nearly doubles

Design data needed for internal corrosion control

  • CO2
  • H2S
  • Oxygen content
  • Operating temperature and pressure
  • Organic Acids, pH
  • Velocity
  • Solids, three-phase flow, scaling

Design data needed for external corrosion control

  • Operating temperature
  • Water temperature
  • Water movement
  • Depth of water
  • Burial
  • Pipe size and length

External corrosion failure modes

  • Seawater corrosion, scowering, abrasion of the coating, and sea bottom movement
  • Galvanic corrosion (dissimilar metals in an electrolyte)
  • Oxygen concentrations cell corrosion (pitting and crevice corrosion)

Design Of Risers Effects The Choice Of Coating Materials

  • Riser can be coated
  • Fusion Bond Epoxies
  • Three-layer systems
  • Rubber linings
  • Risers can have extra wall thickness for corrosion and mechanical damage allowance

Typical Pipeline Coatings For Corrosion Control

  • External pipeline protective coatings
  • Fusion bond epoxy
  • With a rough coat
  • Polyethylene coatings (2 layers)
  • Three-layer
  • Polyolefin coatings (3-layer PE and 3-layer PP)
  • Bitumen, asphalt and coal tar coatings
  • Coal tar epoxies
  • Coal tar urethanes
  • External concrete coatings
  • weight coatings
  • Polymer concrete

Qualification requirements for fusion bonded epoxy coatings

Qualification requirements for fusion bonded epoxy coatings
Fig. 2: Qualification requirements for fusion bonded epoxy coatings

Typical properties for extruded polyolefin coatings

Typical properties for extruded polyolefin coatings
Fig. 3: Typical properties for extruded polyolefin coatings

Typical properties for coal tar enamel coatings

Typical properties for coal tar enamel coatings
Fig. 4: Typical properties for coal tar enamel coatings

Typical corrosion allowances for internal corrosion of carbon steel subject to in-service corrosion:

Typical corrosion allowances for internal corrosion of carbon steel subject to in service corrosion
Fig. 5: Typical corrosion allowances for internal corrosion of carbon steel subject to in-service corrosion

Cathodic Protection of Sub-sea Pipelines

  • Impressed current cathodic protection (ICCP)
  • Onshore
  • Offshore
  • Sacrificial cathodic protection
  • Bracelet anodes
  • Aluminum
  • Zinc

Cathodic protection design parameters and coatings design considerations:

Cathodic protection design parameters and coatings design considerations
Fig. 6: Cathodic protection design parameters and coatings design considerations

Monitoring and Inspection of Coatings

  • As part of the corrosion designs for offshore pipelines, corrosion monitoring and corrosion inspection plans are needed.
  • These plans are intended to monitor the effectiveness of corrosion mitigation and to measure changes as the pipeline ages.

Determining the effectiveness of corrosion control

  • Side scanning sonar detect areas where the pipeline may be bridging the ocean floor
  • shifting of the ocean floor
  • Cathodic protection monitoring
  • potential survey
  • current drain surveys.

These surveys provide information about the condition of the cathodic protection system coating performance and coating breakdown.

Inspection and testing can demonstrate pipeline integrity

In-line inspection tools

  • Magnetic flux leakage
  • Ultrasonic testing

Hydrostatic testing

  • Pressure testing
  • Spike testing

Coatings supplemented with cathodic protection can result in long service life

The first line of defense for an offshore pipeline is the coating on the pipe. Many different types of coating are used for offshore pipelines.  Coatings for offshore must have good resistance to water absorption, cathodic dis-bondment, and strong adhesion to the pipe.

Cathodic protection is used to protect holidays in the coating.

The testing of coatings can provide some guidance concerning the ability of a coating to survive the offshore environment.  Coating history and performance is very valuable information.

Few more Pipeline related useful Resources for You..

Underground Piping Stress Analysis Procedure using Caesar IIOpens in a new tab.
Comparison between Piping and Pipeline EngineeringOpens in a new tab.
A Presentation on Pipelines – Material Selection in Oil & Gas IndustryOpens in a new tab.
Corrosion Protection for Offshore PipelinesOpens in a new tab.
Start up and Commissioning of the Pipeline: An ArticleOpens in a new tab.
DESIGN OF CATHODIC PROTECTION FOR DUPLEX STAINLESS STEEL (DSS) PIPELINEOpens in a new tab.
AN ARTICLE ON MICRO TUNNELING FOR PIPELINE INSTALLATIONOpens in a new tab.
A short presentation on: OFFSHORE PIPELINE SYSTEMS: Part 1Opens in a new tab.
Factors Affecting Line Sizing of Piping or Pipeline SystemsOpens in a new tab.
Few Job Opportunities for YouOpens in a new tab.

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Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

One thought on “Corrosion Protection for Offshore Pipelines

  1. Its very difficult to handle machinery ,equipment and pipelines which are located in offshore.This technology is amazing.Thanks for sharing such amazing and informative article which will help people to make the pipeline corrosion and rust free in the offshore are .

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